Aperture masks for circuit fabrication

ABSTRACT

Aperture masks and deposition techniques for using aperture masks are described. In addition, techniques for creating aperture masks and other techniques for using the aperture masks are described. The various techniques can be particularly useful in creating circuit elements for electronic displays and low-cost integrated circuits such as radio frequency identification (RFID) circuits. In addition, the techniques can be advantageous in the fabrication of integrated circuits incorporating organic semiconductors, which typically are not compatible with wet processes.

This application is a divisional of U.S. Ser. No. 10/076,174, filed Feb.14, 2002, now allowed, the disclosure of which is herein incorporated byreference.

TECHNICAL FIELD

The invention relates to fabrication of circuits and circuit elements,and more particularly to deposition techniques using aperture masks.

BACKGROUND

Circuits include combinations of resistors, diodes, capacitors andtransistors linked together by electrical connections. Thin filmintegrated circuits include a number of layers such as metal layers,dielectric layers, and active layers typically formed by a semiconductormaterial such as silicon. Typically, thin film circuit elements and thinfilm integrated circuits are created by depositing various layers ofmaterial and then patterning the layers using photolithography in anadditive or subtractive process which can include a chemical etchingstep to define various circuit components. Additionally, aperture maskshave been used to deposit a patterned layer without an etching step.

SUMMARY

In general, the invention is directed to deposition techniques usingflexible, repositionable polymeric aperture masks to create integratedcircuits or integrated circuit elements. The techniques involvesequentially depositing material through a number of polymeric aperturemasks formed with patterns that define layers, or portions of layers, ofthe circuit. In some embodiments, circuits can be created solely usingaperture mask deposition techniques, without requiring any of theetching or photolithography steps typically used to form integratedcircuit patterns. The techniques can be particularly useful in creatingcircuit elements for electronic displays such as liquid crystal displaysand low-cost integrated circuits such as radio frequency identification(RFID) circuits. In addition, the techniques can be advantageous in thefabrication of integrated circuits incorporating organic semiconductors,which typically are not compatible with photolithography or other wetprocesses.

In various embodiments, the invention is directed to differentrepositionable aperture masks such as flexible aperture masks,free-standing aperture masks and polymeric aperture masks formed withpatterns that define a layer or a portion of a layer of an integratedcircuit. Repositionable polymeric aperture masks may have a thickness ofapproximately between 5 and 50 microns or approximately between 15 and35 microns. The various deposition apertures in the aperture masks mayhave widths less than approximately 1000 microns, less thanapproximately 50 microns, less than approximately 20 microns, less thanapproximately 10 microns, or even less than approximately 5 microns.Apertures of these sizes are particularly useful in creating smallcircuit elements for integrated circuits. Moreover, one or more gapsbetween deposition apertures may be less than approximately 1000microns, less than approximately 50 microns, less than approximately 20microns or less than approximately 10 microns, which is also useful increating small circuit elements. Also, aperture masks that include apattern having a width greater than approximately 1 centimeter, 25centimeters, 100 centimeters, or even 500 centimeters are alsodescribed. Patterns having these widths can be useful in creatingvarious circuits over a larger surface area as described in greaterdetail below. In some embodiments, the invention is directed to methodsof depositing material on a deposition substrate through therepositionable polymeric aperture masks.

In other embodiments, the invention is directed to various techniquesfor creating or using the masks described above. For example, variouslaser ablation techniques are described that facilitate the creation ofpolymeric aperture masks having the patterns of deposition aperturesdescribed above. In addition, stretching techniques and other techniquesare described to facilitate alignment of flexible polymeric aperturemasks. Furthermore, methods of controlling sag in aperture masks arealso described, which can be particularly useful in using masks thatinclude a pattern that extends over a large width.

The various embodiments of the invention can provide a number ofadvantages. For example, the invention can facilitate the creation ofrelatively small circuit elements using deposition processes. Theinvention can facilitate circuit elements having widths less thanapproximately 1000 microns, less than approximately 50 microns, lessthan approximately 20 microns, less than approximately 10 microns, oreven less than approximately 5 microns. Also, the invention canfacilitate the creation of relatively large circuit patterns, in somecases having circuit elements of the relatively small widths mentionedabove that cover large areas (such as 10 square centimeters, 50 squarecentimeters, 1 square meter, or even larger areas). In addition, theinvention can reduce costs associated with circuit fabrication, and inthe case of organic semiconductors, can even improve device performance.Polymeric aperture masks can be created using a laser ablation processthat may be faster and less expensive than other techniques. Also,inexpensive polymeric materials can allow the polymeric masks to bedisposable, although reusable embodiments are also described.

In addition, polymeric material may be well suited to be impregnatedwith magnetic material. In that case, the magnetic material may be usedto reduce sag in the mask as described below. Furthermore, polymericmaterial is often stretchable, which allows the mask to be stretched toeither reduce sag or to align the mask as outlined below.

Details of these and other embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will become apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 a is a top view of an aperture mask according to an embodiment ofthe invention.

FIG. 1 b is an enlarged view of a portion of the aperture mask in FIG 1a.

FIGS. 2 and 3 are top views of aperture masks according to an embodimentof the invention.

FIG. 4 is a top view of an exemplary mask set according to theinvention.

FIG. 5 is a block diagram of a deposition station using an aperture maskaccording to the invention.

FIG. 6 is a cross-sectional side view of an aperture mask according toan embodiment of the invention.

FIGS. 7 and 8 are additional block diagrams of deposition stations usingan aperture mask according to the invention.

FIG. 9 a is a perspective view of one exemplary stretching unit forstretching aperture masks in accordance with the invention.

FIG. 9 b is an enlarged view of a stretching mechanism.

FIG. 10 is a block diagram of a laser ablation system that can be usedto ablate aperture masks in accordance with the invention.

FIG. 11 is a cross-sectional side view of a polymeric film formed with amaterial on a first side.

FIGS. 12 and 13 are cross-sectional views of exemplary thin filmtransistors that can be created according to the invention.

FIG. 14 is a top view of one embodiment of an aperture mask beingstretched in accordance with the invention.

DETAILED DESCRIPTION

FIG. 1 a is a top view of an aperture mask according to the invention.In exemplary embodiments, aperture mask 10A is formed from a polymermaterial such as polyimide or polyester. However, in some embodiments,where aperture mask 10A is flexible, other flexible non-polymericmaterials may be used. The use of polymeric materials for aperture mask10A can provide advantages over other materials, including ease offabrication of aperture mask 10A, reduced cost of aperture mask 10A, andother advantages. As compared to thin metal aperture masks, polymeraperture masks are much less prone to damage due to the accidentalformation of creases or permanent bends. Furthermore, some polymer maskscan be cleaned with acids.

As shown in FIGS. 1 a and 1 b, aperture mask 10A is formed with apattern 12A that defines a number of deposition apertures 14 (onlydeposition apertures 14A-14E are labeled). The arrangement and shapes ofdeposition apertures 14A-14E in FIG. 1 b are simplified for purposes ofillustration, and are subject to wide variation according to theapplication and circuit layout envisioned by the user. Pattern 12Adefines at least a portion of a circuit layer and may generally take anyof a number of different forms. In other words, deposition apertures 14can form any pattern, depending upon the desired circuit elements orcircuit layer to be created in the deposition process using aperturemask 10. For example, although pattern 12A is illustrated as including anumber of similar sub-patterns (sub-patterns 16A-16C are labeled), theinvention is not limited in that respect.

Aperture mask 10A can be used in a deposition process, such as a vapordeposition process in which material is deposited onto a depositionsubstrate through deposition apertures 14 to define at least a portionof a circuit. Advantageously, aperture mask 10A enables deposition of adesired material and, simultaneously, formation of the material in adesired pattern. Accordingly, there is no need for a separate patterningstep preceding or following deposition. Aperture mask 10A can beparticularly useful in creating circuits for electronic displays,low-cost integrated circuits such as RFID circuits, or any circuit thatimplements thin film transistors. Moreover, circuits that make use oforganic semiconductors can benefit from various aspects of the inventionas described in greater detail below.

One or more deposition apertures 14 can be formed to have widths lessthan approximately 1000 microns, less than approximately 50 microns,less than approximately 20 microns, less than approximately 10 microns,or even less than approximately 5 microns. By forming depositionapertures 14 to have widths in these ranges, the sizes of the circuitelements may be reduced. Moreover, a distance (gap) between twodeposition apertures (such as for example the distance betweendeposition aperture 14C and 14D) may be less than approximately 1000microns, less than approximately 50 microns, less than approximately 20microns or less than approximately 10 microns, to reduce the size ofvarious circuit elements. When making, using, reusing, or repositioningthe aperture masks the distances between features, such as the distancebetween apertures or the distance between sub-patterns may bereproducible to within approximately 1.0 percent, approximately 0.5percent, or even approximately 0.1 percent.

As described in greater detail below, laser ablation techniques may beused to define pattern 12A of deposition apertures 14. Accordingly,formation of aperture mask 10A from a polymeric film can allow the useof fabrication processes that can be less expensive, less complicated,and/or more precise than those generally required for other aperturemasks such as silicon masks or metallic masks. Moreover, because laserablation techniques can be used to create pattern 12A, the width ofpattern 12A can be made much larger than conventional patterns. Forexample, laser ablation techniques can facilitate the creation ofpattern 12A such that a width of pattern 12A is greater thanapproximately one centimeter, greater than approximately 25 centimeters,greater than approximately 100 centimeters, or even greater thanapproximately 500 centimeters. These large masks, which can be the widthof a web, and very long, e.g., the length of a roll, can then be used ina deposition process to create circuit elements that are distributedover a large surface area and separated by large distances.

FIGS. 2 and 3 are top views of aperture masks 10 that include depositionapertures separated by relatively large widths. In particular, FIG. 2illustrates aperture mask 10B, which includes a pattern 12B ofdeposition apertures. Pattern 12B may define at least one dimension thatis greater than approximately one centimeter, 25 centimeters, 100centimeters, or even greater than approximately 500 centimeters. Inother words, the distance X may be within those ranges. In this manner,circuit elements separated by larger than conventional distances can becreated using a deposition process. This feature may be advantageous,for example, in the fabrication of large area flat panel displays ordetectors.

For some circuit layers, complex patterns may not be required. Forexample, aperture mask 10C of FIG. 3 includes at least two depositionapertures 36A and 36B. In that case, the two deposition apertures 36Aand 36B can be separated by a distance X that is greater thanapproximately one centimeter, 25 centimeters, 100 centimeters, or even500 centimeters. Again, laser ablation techniques can facilitate therelatively large distance X because laser ablation systems can be easilydesigned to facilitate patterning over the large areas. Moreover, laserablation techniques can facilitate the creation of deposition apertures36A and 36B to widths less than approximately 1000 microns, less thanapproximately 50 microns, less than approximately 20 microns, less thanapproximately 10 microns, or even less than approximately 5 microns. Inthat case, the deposition process would not necessarily require theaperture mask to be registered or aligned to a tolerance as small as theaperture widths. Still, the ability to deposit a circuit layer in asingle deposition process with elements separated by these largedistances can be highly advantageous for creating circuits that requirelarge separation between two or more elements. Circuits for controllingor forming pixels of large electronic displays are one example.

FIG. 4 is a top view of a mask set 40 that includes a number of aperturemasks 10D-10I according to the invention for use in a depositionprocess. Mask set 40 may include any number of polymeric aperture masks,for example, depending on the circuit or circuit element to be createdin the deposition process. Masks 10D-10I form a “set” in the sense thateach mask may correspond to a particular layer or set of circuitelements within an integrated circuit. Each aperture mask 10 can beformed with a pattern of deposition apertures that define at least partof a layer of a circuit. For example, a first aperture mask 10D may beformed with a first pattern of deposition apertures that define at leastpart of a first deposition layer of a circuit, while a second aperturemask 10E may be formed with a second pattern of deposition aperturesthat define at least part of a second deposition layer of the circuit.The mask set 40 may be used to create a wide variety of integratedcircuits, such as integrated circuits which include both n-channel andp-channel thin film transistors (TFTs), such as a complimentary TFTelement. In addition, organic or inorganic semiconductor materials maybe used to create integrated circuits according to the invention. Forsome circuits, both organic and inorganic semiconductors may be used.

In some cases, the first and second aperture masks 10D and 10E maydefine different layers of a circuit, and in other cases, first andsecond aperture masks 10D and 10E may define different portions of thesame circuit layer. In some cases, stitching techniques can be used inwhich first and second aperture masks 10D and 10E define differentportions of the same circuit feature. In other words, two or more masksmay be used in separate depositions to define a single circuit feature.Stitching techniques can be used, for example, to avoid relatively longdeposition apertures, closed curves or any aperture pattern that wouldcause a portion of the aperture mask to be poorly supported, or notsupported at all. In a first deposition, one mask forms part of afeature, and in a second deposition, another mask forms the remainder ofthe feature.

Each aperture mask 10 in mask set 40 may comprise a polymer. In thatcase, laser ablation techniques can be used to form one or moredeposition apertures having widths less than approximately 1000 microns,less than approximately 50 microns, less than approximately 20 microns,less than approximately 10 microns, or even less than approximately 5microns. In addition, at least two deposition apertures in each aperturemask may be separated by a gap less than approximately 1000 microns,less than approximately 50 microns, less than approximately 20 microns,or less than approximately 10 microns. Apertures and gaps betweenapertures of these dimensions can reduce circuit size, and in some casesimprove circuit performance. Also, a dimension of the pattern ofdeposition apertures of aperture masks 10D-10I in mask set 40 may be inthe ranges mentioned above to facilitate the creation of circuits havinglarge dimensions.

Polymeric aperture masks are useful with a wide variety of materials tobe deposited. For example, organic or inorganic materials may bedeposited through polymeric aperture masks. In one example, amorphoussilicon may be deposited. Deposition of amorphous silicon typicallyrequires high temperatures greater than approximately 200 degreesCelsius. Some embodiments of the polymeric aperture masks describedherein may be able withstand these high temperatures, thus allowingamorphous silicon to be deposited through the polymeric mask to createintegrated circuits or integrated circuit elements.

FIG. 5 is a simplified block diagram of a deposition station that canuse aperture masks in a deposition process according to the invention.In particular, deposition station 50 can be constructed to perform avapor deposition process in which material is vaporized and deposited ona deposition substrate through an aperture mask. The deposited materialmay be semiconductor material such as an organic or inorganicsemiconductor, dielectric material, or conductive material used to forma variety of elements within an integrated circuit. Other materialscould also be used. Flexible aperture mask 10J is placed in proximity toa deposition substrate 52. In one example, flexible aperture mask 10J issufficiently flexible such that it can be wound upon itself. In anotherexample, flexible aperture mask 10J is sufficiently flexible such thatit can be wound to a radius of curvature of less than approximately 1centimeter without damage, or without forming a permanent bend.Deposition substrate 52 may comprise any of a variety of materialsdepending on the desired circuit to be created. For example, depositionsubstrate 52 may comprise a flexible material, such as a flexiblepolymer. Additionally, if the desired circuit is a circuit oftransistors for an electronic display, deposition substrate 52 maycomprise the backplane of the electronic display. Any depositionsubstrates such as glass substrates, silicon substrates, rigid plasticsubstrates, flexible plastic substrates, metal foils coated withinsulating layers, or the like, could also be used.

Deposition station 50 is typically a vacuum chamber. After flexibleaperture mask 10J is placed in proximity to deposition substrate 52,material 56 is vaporized by deposition unit 54. For example, depositionunit 54 may include a boat of material that is heated to vaporize thematerial. The vaporized material 56 deposits on deposition substrate 52through the deposition apertures of flexible aperture mask 10J to defineat least a portion of a circuit layer on deposition substrate 52. Upondeposition, material 56 forms the pattern defined by flexible aperturemask 10J. Flexible aperture mask 10J may include deposition aperturesand gaps that are sufficiently small to facilitate the creation of smallcircuit elements using the deposition process as described above.Additionally, the pattern of deposition apertures in flexible aperturemask 10J may have a large dimension as mentioned above. Other suitabledeposition techniques include e-beam evaporation and various forms ofsputtering and pulsed laser deposition.

However, when flexible aperture mask 100 is made sufficiently large, forexample, to include a pattern that has large dimensions, a sag problemmay arise. In particular, when flexible aperture mask 10J is placed inproximity to deposition substrate 52, flexible aperture mask 10J may sagas a result of the gravitational pull on flexible aperture mask 10J.This problem is most apparent when the aperture mask 10J is positionedunderneath deposition substrate as shown in FIG. 5. Moreover, the sagproblem compounds as flexible aperture mask 10J is made larger andlarger. FIG. 5 illustrates the sag problem that results fromgravitational pull on flexible aperture mask 10J.

The invention may implement one of a variety of techniques to addressthe sag problem or otherwise control sag in aperture masks during adeposition process. For example, FIG. 6 is a cross-sectional side viewof a flexible aperture mask 10K having a first side 61 that canremovably adhere to a surface of a deposition substrate to facilitateintimate contact between the aperture mask and the deposition substrateduring the deposition process. In this manner, sag can be controlled oravoided. In particular, first side 61 of repositionable flexibleaperture mask 10K may include a pressure sensitive adhesive. In thatcase, first side 61 can removably adhere to a deposition substrate viathe pressure sensitive adhesive, and can then be removed after thedeposition process.

FIG. 7 is a block diagram of a deposition system 70 making use offlexible aperture mask 10K illustrated in FIG. 6. As shown, the sagproblem is overcome because first side 61 of flexible aperture mask 10Kadheres to deposition substrate 52. Material 56 can then be vaporized bydeposition unit 54 and deposited onto deposition substrate 52 accordingto the pattern defined in flexible aperture mask 10K. In some cases,flexible aperture mask 10K may be removed and repositioned in order toachieve proper placement of flexible aperture mask 10K relative todeposition substrate 52 for the deposition process.

Another way to control sag is to use magnetic force. For example,referring again to FIG. 1 a, aperture mask 10A may comprise both apolymer and magnetic material. The magnetic material may be coated orlaminated on the polymer, or can be impregnated into the polymer. Forexample, magnetic particles may be dispersed within a polymeric materialused to form aperture mask 10A. When a magnetic force is used, amagnetic field can be applied within a deposition station to attract orrepel the magnetic material in a manner that controls sag in aperturemask 10A.

For example, as illustrated in FIG. 8, a deposition station 80 mayinclude magnetic structure 82. Aperture mask 10L may include a magneticmaterial. Magnetic structure 82 may attract aperture mask 10L so as toreduce, eliminate, or otherwise control sag in aperture mask 10L.Alternatively, magnetic structure 82 may be positioned such that sag iscontrolled by repelling the magnetic material within aperture mask 10L.In that case magnetic structure 82 would be positioned on the side ofaperture mask 10L opposite deposition substrate 52. For example,magnetic structure 82 can be realized by an array of permanent magnetsor electromagnets.

Another way to control sag is the use of electrostatics. In that case,the aperture mask may comprise a polymer that is electrostaticallycoated or treated. Although magnetic structure 82 (FIG. 8) may not benecessary if an electrostatic coating is used to control sag, it may behelpful in some cases where electrostatics are used. A charge may beapplied to the aperture mask, the deposition substrate, or both topromote electrostatic attraction in a manner that promotes a sagreduction.

Still another way to control sag is to stretch the aperture mask. Inthat case a stretching unit can be implemented to stretch the aperturemask by an amount sufficient to reduce, eliminate, or otherwise controlsag. As the mask is stretched tightly, sag is reduced. In that case, theaperture mask may need to have an acceptable coefficient of elasticity.

Additionally, the concept of stretching a polymeric aperture mask canalso be used to properly align the aperture mask for a depositionprocess. FIG. 9 a is a perspective view of an exemplary stretching unitfor stretching aperture masks in accordance with the invention.Stretching unit 90 may include a relatively large deposition hole 92. Anaperture mask can cover deposition hole 92 and a deposition substratecan be placed in proximity with the aperture mask. In order to allowease of alignment using stretching, the aperture mask should allowelastic stretching without damage. Thus, the amount of stretching in oneor more directions may be greater than approximately 0.1 percent,greater than approximately 0.5 percent, or even greater thanapproximately 1 percent. Material can be vaporized through depositionhole 92, and deposited on the deposition substrate according to thepattern defined in the aperture mask.

Stretching unit 90 may include a number of stretching mechanisms 95A,95B, 95C and 95D. Each stretching mechanism 95 may protrude up through astretching mechanism hole 99 shown in FIG. 9 b. In one specific example,each stretching mechanism 95 includes a top clamp portion 96 and abottom clamp portion 97 that can clamp together upon an aperture mask.The aperture mask can then be stretched by moving stretching mechanisms95 away from one another as they clamp the aperture mask. Stretchingmechanisms 95 may move in directions along one or more axes. Stretchingmechanisms 95 are illustrated as protruding from the top of stretchingunit 90, but could alternatively protrude from the bottom of stretchingunit. Particularly, if stretching unit 90 is used to control sag in anaperture mask, the stretching mechanisms would typically protrude fromthe bottom of stretching unit 90. Alternative methods of stretching theaperture mask could also be used either to control sag in the aperturemask or to properly align the aperture mask for the deposition process.By stretching the aperture mask, misalignment due to thermal expansioncan be greatly minimized.

As mentioned above, laser ablation techniques can be used to create thepattern of deposition apertures in a polymeric aperture mask, forexample, by ablating a polymeric film to define the pattern ofdeposition apertures. In some cases, the pattern may define first andsecond pattern elements separated by more than approximately 25centimeters. FIG. 10 is a block diagram of a laser ablation system thatcan be used to ablate aperture masks in accordance with the invention.Laser ablation techniques are advantageous because they can achieverelatively small deposition apertures and can also define patterns on asingle aperture mask that are much larger than conventional patterns. Inaddition, laser ablation techniques may facilitate the creation ofaperture masks at significantly lower cost than other conventionaltechniques commonly used to create metal or silicon aperture masks.

As illustrated in FIG. 10, laser ablation system 100 may be a projectionlaser ablation system utilizing a patterned ablation mask, although ashadow mask ablation system or phase mask ablation system could be used.Spot-writing a single laser spot can also be used to “write” the desiredpattern of apertures. Projection imaging laser ablation is a techniquefor producing very small parts or very small structures on a surface ofan object being ablated, the structures having sizes on the order ofbetween one micron to several millimeters, whereby light is passedthrough a patterned ablation mask and the pattern is imaged onto theobject being ablated. Material is removed from the ablation substrate inthe areas that receive light. Although the system is described using anultraviolet (UV) laser, the illumination provided by the laser can beany kind of light, such as infrared or visible light. Moreover, theinvention may be applied using light sources that are not lasers.

Laser 101 may be a KrF excimer laser emitting a beam with a shortwavelength of light of approximately 248 nm. However, any type ofexcimer laser may be used, such as F₂, ArF, KrCl, or XeCl type excimerlasers. An excimer laser is particularly useful in creating smalldeposition apertures because an excimer laser can resolve smallerfeatures and cause less collateral damage than lasers such as CO₂lasers, which emit beams with a wavelength of approximately 10,600 nm.Also, excimer lasers can be used with most polymers that are transparentto light from lasers typically used for processing metals, such asNeodymium doped Yttrium Aluminum Garnet (Nd:YAG) lasers. Excimer lasersare also advantageous because at UV wavelengths, most materials, such aspolymers, have high absorption. Therefore, more energy is concentratedin a shallower depth and the excimer laser provides cleaner cutting.Excimer lasers are pulsed lasers, the pulses ranging from 5-300nanoseconds. Laser 101 may also be a tripled or quadrupled Nd:YAG laseror any laser having pulses in the femtosecond range.

Ablation mask 103 may be a patterned mask having pattern 102manufactured using standard semiconductor lithography mask techniques.The patterned portions of ablation mask 103 are opaque to UV light,while a support substrate of ablation mask are transparent to UV light.In one embodiment, the patterned portions comprise aluminum while thesupport substrate for ablation mask 103 is fused silica (SiO₂). Fusedsilica is a useful support material because it is one of the fewmaterials that is transparent to mid and far UV wavelengths. Calciumfluoride may be used as the support substrate as an alternative to SiO₂.Aluminum is useful as a patterning material because it reflects mid-UVlight. A patterned dielectric stack is one alternative to aluminum.

Imaging lens 104 may be a single lens or an entire optical systemconsisting of a number of lenses and other optical components. Imaginglens 104 projects an image of the ablation mask, more specifically, animage of the pattern of light passing through the ablation mask ontosurface of object to be ablated 105. The object to be ablated is apolymeric film 106, possibly including a material 107 formed on the backside of the polymeric film. Some suitable polymeric films may comprisepolyimide, polyester, polystyrene, polymethyl methacrylate,polycarbonate, or combinations thereof.

FIG. 11 illustrates a useful structure that can form the object to beablated 105. Specifically, FIG. 11 illustrates an object to be ablated105 that includes a polymer film 106 with a material 107 formed on theback side, i.e. a side opposite the side incident to the laser in system100 (FIG. 10). Material 107 provides an etch stop for the ablationprocess which can avoid air entrapment under polymer film 106, and canbe a stabilizing carrier. For example, material 107 may comprise a metalsuch as copper.

After the ablation is complete, material 107 is etched from polymer film106, to form a polymeric aperture mask. Alternatively, in someembodiments, material 107 may be peeled away. Objects 105 may be createdby forming a copper layer on a polymer film, or by forming the polymerfilm on a copper layer. In some cases, objects 105 may simply bepurchased in a preformed configuration.

Referring again to FIG. 10, table 109 supports and positions the objectto be ablated 105. For example, object to be ablated 105 can be fixedinto position on table 109, such as by vacuum chuck 108, staticelectricity, mechanical fasteners or a weight. Table 109 can positionthe object to be ablated 105 by moving the object 105 on the x, y and zaxes as well as rotationally, such as along the z axis. Table 109 canmove object 105 in steps down to approximately 5 nm, and more typically,approximately 100 nm, reproducible to an accuracy of approximately 500nm. Computer control of table 109 can allow preprogramming of themovement of table 109 as well as possible synchronization of tablemovement with the emission of light from laser 101. The table may alsobe manually controlled, such as with a joystick connected to a computer.

In creating aperture masks for integrated circuit fabrication, it can beadvantageous to control the wall angle of the ablated depositionapertures so that the deposition apertures are suitable for material tobe deposited through them. In an embodiment of the invention, theablation is controlled to obtain an acceptable wall angle. Accordingly,the invention may control the ablation so as to achieve an acceptablewall angle. A straight wall angle, i.e., a zero (0) degree wall angle,corresponds to a deposition aperture having walls that are perpendicularto the surface of the polymer film. In some cases, even a negative wallangle can be achieved, wherein the hole assumes a larger and largerdiameter as the laser ablates through the polymer material.

A number of factors can affect the wall angle. Accordingly these factorscan be controlled to achieve an acceptable, or a desired wall angle. Forexample, the power density of the laser radiation at the substrate andthe numerical aperture of the imaging system can be controlled toachieve an acceptable wall angle. Additional factors that may becontrolled include the pulse length of the laser, and the ablationthreshold of the object or material being ablated. In general, theaperture wall angle should be near zero to allow the closest possiblespacing between apertures. However, if a large aperture mask is used ina deposition process with a small source, e.g., electron beamevaporation, a wall angle greater than zero is desirable to minimizeparallax in regions of the mask where the deposition flux impinges thedeposition substrate at an angle substantially different fromperpendicular.

FIGS. 12 and 13 are cross-sectional views of exemplary thin filmtransistors that can be created according to the invention. Inaccordance with the invention, thin film transistors 130 and 140 can becreated without using any etching or lithography techniques. Instead,thin film transistors 130 and 140 can be created solely using aperturemask deposition techniques as described herein. Alternatively, one ormore bottom layers may be etched or lithographically patterned, with atleast two of the top most layers being formed by the aperture maskdeposition techniques described herein. Importantly, the depositiontechniques achieve sufficiently small circuit features in the thin filmtransistors. In addition, if an organic semiconductor is used, theinvention can facilitate the creation of thin film transistors in whichthe organic semiconductor is not the top-most layer of the circuit.Rather, in the absence of wet processing, electrode patterns may beformed over the organic semiconductor material. Electrode patternsformed over the organic semiconductor often result in thin filmtransistors with improved device characteristics. This advantage ofaperture mask 10 can be exploited while at the same time achievingacceptable sizes of the circuit elements.

Thin film transistors are commonly implemented in a variety of differentcircuits, including for example, RFID circuits, electronic memory, andother low cost circuits. In addition, thin film transistors can be usedas control elements for liquid crystal display pixels, or other flatpanel display pixels, including organic light emitting diodes. Manyother applications for thin film transistors also exist.

As shown in FIG. 12, thin film transistor 130 is formed on a depositionsubstrate 131. Thin film transistor 130 represents one embodiment of atransistor in which all of the layers are deposited using an aperturemask and none of the layers are formed using etching or lithographytechniques. The aperture mask deposition techniques described herein canenable the creation of thin film transistor 130 in which a distancebetween electrodes 135 and 136 is less than approximately 1000 microns,less than approximately 50 microns, less than approximately 20 microns,or even less than approximately 10 microns, while at the same timeavoiding conventional etching or lithography processes.

In particular, thin film transistor 130 includes a first depositedconductive layer 132 formed over deposition substrate 131. A depositeddielectric layer 133 is formed over first conductive layer 132. A seconddeposited conductive layer 134 defining source electrode 135 and drainelectrode 136 is formed over deposited dielectric layer 133. A depositedactive layer 137, such as a deposited semiconductor layer, or adeposited organic semiconductor layer is formed over second depositedconductive layer 134. Deposition techniques using mask set 40, forexample, represent one exemplary method of creating thin film transistor130. In that case, each layer of thin film transistor 130 may be definedby one or more deposition apertures in deposition aperture masks 10D-10Ithat form mask set 40. Alternatively, one or more of the layers of thethin film transistor may be created using a number of aperture masks andstitching techniques, as mentioned above.

By forming deposition apertures 14 in masks 10 to be sufficiently small,one or more features of thin film transistor 130 can be made with awidth less than approximately 1000 microns, less than approximately 50microns less than approximately 20 microns, less than approximately 10microns, or even less than approximately 5 microns. Moreover, by forminga gap in an aperture mask to be sufficiently small, other features suchas the distance between source electrode 135 and drain electrode 136 canbe made less than approximately 1000 microns, less than approximately 50microns less than approximately 20 microns or even less thanapproximately 10 microns. In that case, a single mask may be used todeposit second conductive layer 134, with each of the two electrodes135, 136 being defined by deposition apertures separated by asufficiently small gap. In this manner, the size of thin film transistor130 can be reduced, enabling fabrication of smaller, higher densitycircuitry while maintaining the performance of thin film transistor 130.Additionally, a circuit comprising two or more transistors can be formedby an aperture mask having two deposition apertures separated by a largedistance, as illustrated in FIGS. 2 and 3.

FIG. 13 illustrates another embodiment of a thin film transistor 140. Inparticular, thin film transistor 140 includes a first depositedconductive layer 142 formed over deposition substrate 141. A depositeddielectric layer 143 is formed over first conductive layer 142. Adeposited active layer 144, such as a deposited semiconductor layer, ora deposited organic semiconductor layer is formed over depositeddielectric layer 143. A second deposited conductive layer 145 definingsource electrode 146 and drain electrode 147 is formed over depositedactive layer 144.

Again, by forming deposition apertures 14 in masks 10 to be sufficientlysmall, one or more features of thin film transistor 140 can have widthson the order of those discussed herein. Also, by forming a gap in anaperture mask to be sufficiently small, the distance between sourceelectrode 146 and drain electrode 147 can be on the order of the gapsdiscussed herein. In that case, a single mask may be used to depositsecond conductive layer 145, with each of the two electrodes 146, 147being defined by deposition apertures separated by a sufficiently smallgap. In this manner, the size of thin film transistor 140 can bereduced, and the performance of thin film transistor 140 can beimproved.

For example, thin film transistors implementing organic semiconductorsmay take the form of FIG. 12 because organic semiconductors typicallycannot be etched or lithographically patterned without damaging ordegrading the performance of the organic semiconductor material. Forinstance, morphological changes can occur in an organic semiconductorlayer upon exposure to processing solvents. For this reason, fabricationtechniques in which the organic semiconductor is deposited as a toplayer are commonly used.

By forming at least the top two layers of the thin film transistor usingaperture mask deposition techniques, the invention facilitates theconfiguration of FIG. 13, even if active layer 144 is an organicsemiconductor layer. The configuration of FIG. 13 can promote improvedgrowth of the organic semiconductor layer by allowing the organicsemiconductor layer to be deposited over the relatively flat surface ofdielectric layer 143, as opposed to being deposited over thenon-continuous second conductive layer 134 as illustrated in FIG. 12.For example, if the organic semiconductor material is deposited over anon-flat surface, growth can be inhibited. Thus, to avoid inhibitedorganic semiconductor growth, the configuration of FIG. 13 may bedesirable. In some embodiments, all of the layers may be deposited asdescribed above. Also, the configuration of FIG. 13 is advantageousbecause depositing appropriate source and drain electrodes on theorganic semiconductor provides low-resistance interfaces. Additionally,circuits having two or more transistors separated by a large distancecan also be created, for example, using masks like those illustrated inFIGS. 2 and 3.

An additional advantage of this invention is that an aperture mask maybe used to deposit a patterned active layer which may enhance deviceperformance, particularly in cases where the active layer comprises anorganic semiconductor, for which conventional patterning processes areincompatible. In general, the semiconductor may be amorphous (e.g.,amorphous silicon) or polycrystalline (e.g., pentacene).

One particular technique for creating a circuit or a circuit elementincludes positioning an aperture mask. For example, the mask may bepositioned in proximity to a deposition substrate. In some cases, themask may be placed in intimate contact with the deposition substrate,and in other cases, it can be advantageous to maintain a small gapbetween the deposition substrate and the aperture mask. The aperturemask can then be stretched. Stretching the aperture mask can achieve oneor more of a number of different goals. For example, stretching theaperture mask can reduce sag in the mask. Alternatively or additionally,stretching the aperture mask can align the mask for the depositionprocess. After stretching the aperture mask, material can be depositedthrough the aperture mask onto the deposition substrate to form a layeron the deposition substrate. The layer may comprise a layer in anintegrated circuit, including, for example, a layer in a thin filmtransistor, a diode, or a radio frequency identification circuit. Thediode may be a light emitting diode, including an organic light emittingdiode.

Another technique of creating a circuit or a circuit element includespositioning an aperture mask and controlling sag in the aperture mask.The aperture mask may be positioned in proximity to a depositionsubstrate. Again, in some cases, the mask may be placed in intimatecontact with the deposition substrate, and in other cases, it can beadvantageous to maintain a small gap between the deposition substrateand the aperture mask. Controlling sag can be performed in a number ofways including using magnetic force, electrostatics, stretchingtechniques, or adhering the mask to the deposition substrate such aswith a pressure sensitive adhesive.

In one technique for creating an aperture mask, a material layer isformed on a first side of a polymeric film. Alternatively, the polymericfilm may be formed on the material. The material may comprise a metalsuch as copper. Once formed, the polymeric film can be ablated from aside opposite the material layer, and the material layer can be removed.For example, if the material layer is metallic, it can be removed byetching or peeling away the metal. In this manner, a polymeric aperturemask can be fabricated.

A repositionable polymeric aperture mask can also be used as a patternin an etching process, including a process to etch at least one layer ofa thin film transistor. Then, the same repositionable polymeric aperturemask can be reused as a pattern in another etching process. Thistechnique can simplify a large scale repeated etching process, and alsoreduce the cost of implementing the same etching process a number oftimes.

In another technique, if an aperture mask is made flexible, it canfacilitate the creation of integrated circuits on a non-planardeposition substrate. A flexible and repositionable aperture mask can bepositioned over the non-planar deposition substrate. Then, a layer or aportion of a layer of an integrated circuit can be formed on thenon-planar deposition substrate. The layer of the integrated circuit maycomprise at least a portion of a thin film transistor. The aperturemasks described herein can be particularly useful when the depositionprocess requires intimate contact between the aperture mask and thedeposition substrate. In that case, flexible polymeric aperture maskscan conform to the surface of the non-planar substrate to facilitate theintimate contact.

In another technique for creating an aperture mask, a pattern is ablatedin a polymeric film to create a free-standing aperture mask. In thatcase, ablation can be controlled to create an acceptable wall-angle. Forexample, a number of factors that can affect the wall angle are listedabove, such as the power density of the laser, the numerical aperture ofthe imaging system, the pulse length of the laser, and the ablationthreshold of the object or material being ablated. One or more of thesefactors can be controlled as desired to ensure that an acceptable wallangle is achieved.

FIG. 14 is a top view of an aperture mask being stretched in accordancewith the invention. Aperture mask 150 comprises a mask substrate formedwith pattern 151 as described above. As illustrated, aperture mask 150may include extension portions 152A-152D of the mask substrate that canbe used to stretch aperture mask 150 and to improve uniformity of thestretching of pattern 151 without distortion. Each extension portion 152may include a set of distortion minimizing features, such as slits (onlydistortion minimizing features 154 labeled), which may be located nearthe edge of pattern 151. The distortion minimizing features 154 canfacilitate more precise stretching of aperture mask 150 by reducingdistortion of pattern 151 during stretching. Various configurations ofdistortion minimizing features include slits in the mask substrate,holes in the mask substrate, perforations in the mask substrate, reducedthickness areas in the mask substrate, and the like.

Clamps 156A-156D can be mounted on extension portions 152 of aperturemask 150. Each clamp 156 may be attached to one or more wires, strings,or the like. In FIG. 14, each clamp 156 includes two strings, thusproviding a total of eight degrees of freedom during stretching. Thestrings can be attached to micrometers mounted on an alignment backingstructure. Tension in the strings can be adjusted to provide positioningand a desired amount of stretching of aperture mask 150. In this manner,mask 150 can be aligned with deposition substrate 160.

EXAMPLE 1

In this example, organic integrated circuits were fabricated using fourvacuum deposition steps, and four stretched polymer aperture masks. Theprocess used no photolithography and no wet processing.

All four polymer aperture masks were made with the same technique, whichinvolved laser ablation of apertures in 25 micron thick sheets ofpolyimide with copper backing that was approximately 18 microns thick.After the laser ablation, the copper was removed in an acid etchant, 10HNO₃: 1 HCl, and the mask was rinsed and cleaned. In each mask, thepatterned region was approximately 4.5 cm×4.5 cm.

Four layers were patterned using the aperture masks including: (1) gatemetal, (2) insulator (dielectric), (3) semiconductor, and (4)source/drain metal. Interconnections between circuit components weremade by stitching traces in the gate and source/drain layers. In thisexample, the integrated circuit was designed with minimum line widths of15 microns.

Each of the masks was mounted between four clamps as shown in FIG. 14.Stretching and alignment were accomplished using the eight strings thatwere attached to the ends of clamps 156. Each of the strings wasconnected to a micrometer mounted on an alignment structure. Each clampallowed tension to be applied uniformly along one side of the mask. Byusing at least five (in this case eight) or more degrees of freedom, theelasticity of the mask was used to achieve excellent alignment over theentire patterned area.

In order to reduce distortion of the mask, distortion minimizingfeatures 154, in the form of slits, were cut through the mask on eachside between the patterned region and the clamp. The slits ran parallelto the direction of tension applied by the adjacent clamp 156, and inthis example were spaced approximately 2.5 mm apart from each other,although the invention is not limited in that respect. The slits,located near the patterned region, allowed the patterned region of themask to stretch in the direction perpendicular to the slits with verylittle constraint by clamps 156. In other words, the patterned regionwas allowed to stretch uniformly in all directions.

In this example, a 50 mm×75 mm×0.7 mm float glass deposition substratewas placed in contact with the first mask with minimal downward force soas to minimize stiction and friction. The first mask was stretched about0.5% in both directions. The mask, stretching mechanisms and substratewere placed in a vacuum system and the first layer, 50 nm of Pd, wasdeposited by ion beam sputtering. Electron beam evaporation could alsobe used.

The assembly was removed and the first mask was replaced with the secondmask. The second mask was stretched and aligned to features of the firstlayer on the substrate. The assembly was placed in a vacuum system andthe second layer, 200 nm of Al₂O₃, was deposited by electron-beamevaporation. Sputtering could also be used.

The assembly was again removed and the second mask was replaced with thethird mask. The third mask was stretched and aligned to features of thefirst layer on the substrate. The assembly was placed in a vacuum systemand the third layer, 50 nm of pentacene, was deposited by thermalevaporation.

The assembly was removed again and the third mask was replaced with thefourth mask. The fourth mask was stretched and aligned to features ofthe first layer on the substrate. The assembly was placed in a vacuumsystem and the fourth layer, 150 nm of Au, was deposited by thermalevaporation. Sputtering or electron beam evaporation could also be used.

Finally, the assembly was removed from the vacuum system and thesubstrate was removed from the assembly. At this point, the resultantintegrated circuit was tested and shown to be functional.

EXAMPLE 2

Electronic displays consisting of subpixels of red, green and blueorganic light emitting diodes are also enabled by the use of a stretchedpolymer mask. The driving circuitry for the Organic Light Emitting Diode(OLED) subpixels is provided on the substrate, and can be either activematrix or passive matrix, both of which are known in the art. Thedriving circuitry includes electrodes (for example, indium tin oxideanodes) for the OLED subpixels. The substrate may also include spacersof, for example, photoresist of height 0.1 to 10 microns which hold theaperture mask away from the substrate surface to prevent damage tomaterials on the substrate when the mask is moved.

In this example, the mask pattern is a series of rectangular aperturesin a rectangular array, formed by laser ablation. The apertures are, forexample, 100 microns square, and are spaced 250 microns center-to-centerin both dimensions. The OLED subpixels are made by optionally firstdepositing a buffer layer, such as polythiophene (e.g., Baytron P fromBayer) over all of the electrodes by spin coating. Alternatively, bufferlayers may be vacuum deposited. Next, a hole transport layer, such as 40nm NPD (N,N′-Di(naphthalen-1-yl)-N,N′diphenylbenzidine) is vacuumevaporated over the buffer layer.

Next, the aperture mask is stretched as in the previous example andaligned to the electrodes for the red subpixels. The redelectroluminescent layer is then deposited through the aperture mask.This layer may be, for example, 10 nm of a mixture of 10 weight percentPtOEP (platinum octaethylporphyrin) in CBP(4,4′-Bis(carbazol-9-yl)biphenyl), formed by simultaneously evaporationPtOEP and CBP from two sources.

The aperture mask is then moved, possibly while still in the vacuum, tobe aligned with the electrodes for the green subpixels. The greenelectroluminescent layer is then vacuum deposited through the aperturemask. This layer may be, for example, 10 nm of a mixture of 10 weightpercent Ir(ppy)₃ (tris(2-phenylpyridine)iridium) in CBP. The mask isthen moved, possibly while still in the vacuum, to be aligned with theelectrodes for the blue subpixels. The blue electroluminescent layer isthen vacuum deposited through the aperture mask. This layer may be, forexample, 10 nm CBP. Optionally, a blue dopant, such as perylene (10weight percent in CBP), may be used.

Next, the mask is removed, possibly while still in the vacuum. Anelectron transporting layer is then vacuum deposited. This layer may be,for example, 50 nm of BAlq((1,1′-Bisphenyl-4-Olato)bis(2-methyl-8-quinolinolato)Aluminum).Finally, in the case of an active matrix display, a cathode is depositedover all of the subpixels. The cathode may be, for example, 0.5 nm ofLiF followed by 200 nm of Al. In the case of a passive matrix display,the cathodes must be patterned into rows, typically by using anothershadow mask.

A number of embodiments of the invention have been described. Forexample, a number of different structural components and differentdeposition techniques have been described. The deposition techniques canbe used to create various different circuits solely using deposition,avoiding any chemical etching processes or photolithography, which isparticularly useful when organic semiconductors are involved.Nevertheless, it is understood that various modifications can be madewithout departing from the spirit and scope of the invention. Forexample, although some aspects of the invention have been described foruse in a thermal vapor deposition process, the techniques and structuralapparatuses described herein could be used with any deposition processincluding sputtering, thermal evaporation and electron beam evaporationand pulsed laser deposition. Thus, these other embodiments are withinthe scope of the following claims.

1. A polymeric aperture mask, the polymeric aperture mask comprising: apolymer, and a magnetic material.
 2. The polymeric aperture mask ofclaim 1, wherein the magnetic material is impregnated into the polymer.3. The polymeric aperture mask of claim 1, wherein the magnetic materialis coated on the polymer.
 4. The polymeric aperture mask of claim 1,wherein the magnetic material is laminated to the polymer.
 5. A methodcomprising: positioning a flexible repositionable polymeric aperturemask in proximity to a deposition substrate; controlling sag in theflexible aperture mask; and depositing material through the flexibleaperture mask to form a layer on the deposition substrate that definesat least a portion of an integrated circuit.
 6. The method of claim 5,wherein the flexible aperture mask comprises a patterned polymeric filmimpregnated with magnetic material, and wherein controlling sagcomprises applying a magnetic field to attract or repel the magneticmaterial in a manner that reduces sag in the flexible aperture mask. 7.The method of claim 5, wherein controlling sag comprises applying astatic charge to the flexible aperture mask and electrostaticallyattracting or repelling the charged flexible aperture mask in a mannerthat reduces sag.
 8. The method of claim 5, wherein the flexibleaperture mask includes a pressure sensitive adhesive on one side,wherein controlling sag comprises adhering the flexible aperture mask tothe deposition substrate via the pressure sensitive adhesive.
 9. Themethod of claim 5, wherein controlling sag comprises stretching theflexible aperture mask.
 10. A method comprising: ablating a polymericfilm formed with a material layer on a first side of the polymeric filmfrom a side opposite the material layer to define a pattern in thepolymeric film, wherein the pattern defines deposition apertures thatextend through the polymeric film; and removing the material to form apolymeric aperture mask.
 11. The method of claim 10, wherein ablatingthe polymeric film comprises ablating a polyimide film.
 12. The methodof claim 10, wherein the material layer comprises a metal layer.
 13. Themethod of claim 12, wherein the metal layer comprises a copper layer.14. The method of claim 12, wherein removing the material layercomprises etching the metal layer from the polymeric film.
 15. A methodcomprising: forming a repositionable polymeric aperture mask; using thepolymeric aperture mask as a pattern in an etching process to etch atleast one layer of a thin film transistor.
 16. The method of claim 15,further comprising reusing the polymeric aperture mask as a pattern inanother etching process.
 17. A repositionable polymeric aperture maskcomprising a pattern of deposition apertures that define at least aportion of an integrated circuit, wherein the pattern has a dimensiongreater than a centimeter, wherein at least one deposition aperture hasa width less than approximately 1000 microns and wherein the portion ofthe integrated circuit comprises at least a portion of a thin filmtransistor.
 18. A repositionable polymeric aperture mask comprising: apolyimide; and a pattern of deposition apertures, wherein the patternhas a dimension greater than a centimeter, and wherein at least onedeposition aperture has a width less than approximately 1000 microns.19. A method comprising: ablating a pattern in a polymeric film tocreate a repositionable polymeric aperture mask, wherein the patterndefines at least one deposition aperture that corresponds to an elementof an integrated circuit.
 20. The method of claim 19, further comprisingdepositing material on a deposition substrate through the polymericaperture mask to define a patterned layer of an integrated circuit. 21.The method of claim 19, wherein ablating comprises laser ablating. 22.The method of claim 21, further comprising controlling the laserablation to create an acceptable wall-angle of one or more of thedeposition apertures.